The present invention generally relates to charged particle beam exposure methods and apparatuses, and more particularly to a charged particle beam exposure method which draws a pattern by deflecting a charged particle beam when a stage moves continuously and to a charged particle beam exposure apparatus which employs such a charged particle beam exposure method.
The integration density and functions of semiconductor integrated circuits (ICs) are continuously improved and the IC technology is expected to form the core technology in technological progress for industry in general, including technical fields such as computers, communication systems and mechanical control systems. The integration density of the IC has increased by four times in the last couple of years, and in the case of a dynamic random access memory (DRAM), for example, the memory capacity has improved from 1 Mbit, 4 Mbit, 16 Mbit, 64 Mbit and 256 Mbit to 1 Gbit.
Such high integration density of the IC was made possible by improved technique in forming fine patterns. Due to improvement in the photolithography technique, it has become possible to form a pattern having a size of only 0.5 .mu.m. However, there is a limit in reducing the size of the pattern by the photolithography technique, and a pattern having a size of approximately 0.4 .mu.m is the smallest pattern that can be formed by the photolithography technique. For this reason, it is becoming increasingly difficult to guarantee an accuracy of 0.15 .mu.m or less when forming windows for contact holes and aligning a pattern to an underlayer.
The charged particle beam exposure method was developed as an exposure method which can form a pattern which is finer than that formed by the photolithography technique. This charged particle beam exposure uses a charged particle beam typified by an electron beam. According to the charged particle beam exposure method, it is possible to form a finer pattern at a higher speed and with a higher reliability when compared to the photolithography technique. However, there are demands to further improve the accuracy of the charged particle beam exposure method.
In a charged particle beam exposure apparatus, a charged particle beam is deflected depending on a drawing pattern which is related to a pattern which is to be drawn on a substrate. The pattern is drawn on the substrate which is placed on a stage by irradiating the deflected charged particle beam on the substrate. The method of driving the stage can be divided into a step-and-repeat method and a continuously moving method.
According to the step-and-repeat method, the stage is stationary until the drawing of the pattern for one cell ends. The stage is moved when starting to drawn the pattern for the next cell. The patterns are drawn by repeating the operations of stopping the stage to draw one pattern and moving the stage before drawing the next pattern. But it takes considerable time to repeat the operations of stopping and moving the stage. On the other hand, according to the continuously moving method, the pattern is drawn while continuously moving the stage, and the drawing of the pattern is not stopped while the stage moves. In addition, it is unnecessary to align and connect the patterns between adjacent exposure regions as in the case of the step-and-repeat method because the pattern is drawn continuously. Therfore, compared to the step-and-repeat method, this continuously moving method can improve the speed and the accuracy of the exposure process. For this reason, the continuously moving method is the more desirable method for driving the stage.
When the substrate is placed on the stage, the coordinate system on the stage and the coordinate system on the substrate usually do not match. The main reasons for the difference in the two coordinate systems are the positioning error, which occurs when the substrate is placed on the stage, and the error in the position of a reference mark (or alignment marker), which is provided on the substrate. This reference mark is provided to measure the rotation of the substrate relative to the stage when the substrate is placed on the stage.
The positioning error which occurs when the substrate is placed on the stage is inevitable due to the error in the dimensions of a holder which fixes the substrate on the stage, because the dimension error and the like are inevitably introduced during the production process. In addition, the error in the position of the reference mark inevitably occurs when the reference mark is formed on the substrate. Accordingly, when carrying out the exposure process, it is necessary to correct the amount of deflection of the charged particle beam by taking into consideration the difference in the two coordinate systems.
For example, the patterns are drawn by assuming that chips CH.sub.1 through CH.sub.8 shown in FIG. 1 (A) are arranged on a substrate. Each of the chips CH.sub.1 through CH.sub.8 are a collection of cells CE shown in FIG. 1 (B). A column of cells CE along the stage moving direction is called a frame F.
For example, each cell CE has a size of approximately 2 mm square, and corresponds to a range in which a main deflector of the electron beam exposure apparatus can deflect the electron beam. The cell CE is a collection of sub fields SF shown in FIG. 1 (C). For example, each sub field SF has a size of approximately 100 .mu.m square, and corresponds to a range in which a sub deflector of the electron beam exposure apparatus can deflect the electron beam. One row made up of the sub fields SF arranged in a direction perpendicular to the stage moving direction as shown in FIG. 1 (B) is called a band B.
As shown in FIG. 1 (C), the sub field SF is made up of patterns P.sub.1 through P.sub.4. In this example, there are four patterns, but it is of course possible to include an arbitrary number of patterns in the sub field SF. Each of the patterns P.sub.1 through P.sub.4 are made up of a plurality of shots S. For example, the shot S has a maximum size of approximately 3 .mu.m square, and the size of the shot S can be varied arbitrarily by a slit deflector of the electron beam exposure apparatus.
FIG. 2 (A) shows a case where the coordinate system of the substrate such as that shown in FIG. 1 matches the coordinate system of the stage. FIG. 2 (A) shows a drawable range R.sub.1 on the stage, in which the electron beam can be deflected by the main deflector, and a cell CE.sub.1 on the substrate. In this case, a stage moving direction M matches the Y-axis of the coordinate system. A deflection data (vector) D.sub.3 of the main deflector can be obtained from a vector sum of a deflection data (vector) D.sub.1 from a column center position O.sub.1 of the drawable range R.sub.1 to a center position CO.sub.1 of the cell CE.sub.1, and a deflection data (vector) D.sub.2 which indicates a position coordinate of the main deflector of the sub field to be drawn from the cell center position CO.sub.1.
FIG. 2 (B) shows a case where the coordinate system of the substrate such as that shown in FIG. 1 does not match the coordinate system of the stage. In this case, if a wafer which is used as the substrate is relatively small and the difference between the two coordinate systems is relatively small, all of the main fields in each column fall within the drawable range R.sub.1 as shown in FIG. 3 (A) as the stage moves in the stage moving direction M. Hence, it may be regarded that the electron beam exposure can be carried out with a high accuracy if the amount of deflection of the electron beam is corrected by taking into consideration the difference between the two coordinate systems.
However, even when the difference between the two coordinate systems is relatively small, there is a problem in that not all main fields in each column fall within the drawable range R.sub.1 as the stage moves in the stage moving direction M as shown in FIG. 3 (B) if the substrate is relatively large. In other words, due to the difference between the two coordinate systems, there exists main fields for which the exposure process cannot be carried out, and as a result, the efficiency of the exposure process becomes poor. In the case where the difference between the two coordinate systems is relatively large, it only becomes possible to carry out the exposure process with respect to an extremely limited number of main fields of the substrate.